The present invention relates to a centerless grinder and in particular to a method and apparatus for grinding blanks of a brittle material into barrel shapes.
A typical nuclear reactor fuel element is formed of an elongated cylindrical jacket or cladding of a material such as zirconium or stainless steel surrounding a column of many shorter cylindrical fuel pellets stacked end to end. The pellets are predominantly oxide ceramic materials produced by the compression of powders in a cylindrical mold. The pellets are then sintered to obtain the desired density, and must subsequently be ground to the final desired shape. Fuel pellets manufactured in this manner tend to be extremely brittle and are subject to chipping during the grinding operation.
Recent technological advancements indicate that it is most desirable to have the fuel pellets formed in a barrel shape. However, difficulty has been experienced in grinding cylindrical blanks of a brittle material, such as the oxide ceramics of nuclear fuel, into barrel-shaped pellets in a continuous yet simple manner.
Smith, U.S. Pat. No. 2,579,520 describes a representative method of grinding barrel-shaped pieces. The grinding wheel and the regulating wheel in the Smith specification have surfaces that mate to form an opening in the shape of the cross section of the desired pellet. To feed the blanks into the grinder, either the grinding wheel or the work wheel is slid outward to allow the blanks to be dropped into the grinder. Such a stepwise infeed operation is too slow and cumbersome to be economical for the manufacture of nuclear fuel pellets.
The continuous through-feed centerless grinders of the prior art have also proven unsuitable for grinding barrel-shaped pellets from cylindrical blanks of brittle material. In centerless grinders of this type, blanks are continuously fed parallel to the axis of a grinding wheel through an inlet guide into the gap between the grinding wheel and the regulating wheel and are collected in an outlet guide at the other side. By misaligning the guide in the direction of the regulating wheel, a barrel-shaped pellet can be formed from the cylindrical blanks. However, experience has shown that cylindrical blanks of brittle material are subject to frequent and excessive chipping when ground in such a grinder. Therefore, this method and apparatus, although providing a continuous operation, is unacceptable for manufacturing nuclear fuel pellets.